This file is indexed.

/usr/share/X11/xkb/symbols/parens is in xkb-data 2.23.1-1ubuntu1.

This file is owned by root:root, with mode 0o644.

The actual contents of the file can be viewed below.

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// swap parentheses and square brackets
partial default
xkb_symbols "swap_brackets" {
    replace key <AE09> {        [         9,    bracketleft     ]       };
    replace key <AE10> {        [         0,    bracketright    ]       };
    replace key <AD11> {        [ parenleft,    braceleft       ]       };
    replace key <AD12> {        [ parenright,   braceright      ]       };
};